You can make use of the following guideline to send a PDF to your customers.
Create a PDF
1. You can create a link with a PDF file on some free platforms such as pdfhost.io
To do this you must load the file that you need to convert into a URL.
2. Once the selected file is converted, the URL is created. 
3. You can shorten this link in Short.io, for which you can check the corresponding article.
4. Once the URL is ready, you can add it to the message.
Related links
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How to Create a Message to Request a Review?